In magnetic tape production, a calendering step is required to smooth and flatten the magnetic coating on a base film. The calendering step involves multi-roll assemblies consisting of alternating steel and complaint rolls. The steel rolls are typically chrome plated and polished to a finish of less than 1 microinch AA. The steel rolls are generally heated during operation by means of a heat transfer fluid circulated through the interior of the rolls.
The compliant rolls used in the calendering step are generally prepared from a steel shaft with a relatively soft cover. Cover materials typically used for compliant rolls in such a calendering operation include nylon 6 and compressed paper. These materials have demonstrated a combination of hardness, machinability and surface finish that make them useful in calendering operations. However, each has demonstrated certain disadvantages. For example, while nylon can be polished to a satisfactory surface finish, its maximum operating temperature is about 60.degree. to 70.degree. C., above which subsurface melting and surface blistering can occur. While paper can be operated at somewhat higher temperatures, the best surface finish obtainable is approximately 6 microinches AA. In addition, paper roll covers are somewhat more difficult to assemble.
Polyimide resin, when formed to a tubular configuration, provides a roll cover material which combines the advantages of nylon and paper roll covers previously used, but without the disadvantages of each that had been encountered. Such polyimide roll covers can operate above the temperature limit of other polymeric materials without significant changes in properties. In addition, polyimides exhibit a hardness comparable to nylon and can be polished as smoothly as nylon covers.
Despite these inherent advantages to polyimide roll covers, and despite shrink fit assembly techniques with a metal shaft, rolls covered with polyimide resin, in operation, exhibit slippage between the cover and shaft. Slippage is particularly undesirable in calender roll operations, since the debris generated at the interface between the shaft and the cover may be a source of contamination in the process.